Gases for low damage selective silicon nitride etching

ABSTRACT

Silicon nitride plasma etching processes are disclosed that minimize the SiN roughness layer on a substrate having a SiN layer thereon by simultaneously introducing an oxidizer at a predetermined flow rate and an etch gas into a plasma reaction chamber containing the substrate. The etch gas has the formula C x H y F z , wherein x is 2-5, z is 1 or 2, 2x+2=y+z, and a fluorine atom is located on a terminal carbon atom of the etch gas.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. ProvisionalApplication Ser. No. 62/033,974 filed Aug. 6, 2014, herein incorporatedby reference in its entirety for all purposes.

TECHNICAL FIELD

Silicon nitride plasma etching processes are disclosed that minimize theSiN roughness layer on a substrate having a SiN layer thereon bysimultaneously introducing an oxidizer at a predetermined flow rate andan etch gas into a plasma reaction chamber containing the substrate. Theetch gas has the formula C_(x)H_(y)F_(z), wherein x is 2-5, z is 1 or 2,2x+2=y+z, and a fluorine atom is located on a terminal carbon atom ofthe etch gas.

BACKGROUND

Scaling of transistor gates to smaller dimensions requires processing ofthinner gate stack side-wall insulating layers. Such insulating layers,referred to herein as gate spacer layers, are typically composed ofsilicon nitride. These layers are typically applied conformally to thesubstrate and then patterned using subtractive wet or dry etchprocesses. The process must be highly selective to the substratematerial and etch the insulating layer anisotropically. Theserequirements make dry etching the preferred method since wet etching istypically isotropic.

U.S. Pat. No. 4,529,476 to Kawamoto et al. disclose a dry etching gassuitable for selective etching of silicon nitride and a process forselectively dry-etching silicon nitride with the dry-etching gasconsisting of C, H, and F atom species and having a ratio of F to H byatom of not more than 2.

Reyes-Betanzo et al. disclose higher oxide and nitride film roughnessresults for oxygen-free plasma etch processes using CF₄ or SF₆ etchgases (Vac. Sci. Technol. A 17 (6) 3179 (1999)).

U.S. Pat. No. 6,117,791 to Ko et al. and WO2002/03439 to Micron disclosethe use of fluoroethane to selectively etch doped SiO from undoped SiOand SiN. Thus, these applications promote SiN as an etch stop forfluoroethane.

US2011/0068086 to Suzuki et al. and US2013/105916 to Chang et al.disclose anisotropic silicon nitride etch processes providingselectivity to silicon and silicon oxide using fluorohydrocarbon gaseshaving the composition C_(x)H_(y)F_(z), wherein x is an integer selectedfrom 3, 4, 5, and 6; y and z are positive integers; and y is greaterthan z.

It is well known in the art that SiN/SiO₂ and SiN/Si selectivityincreases as the ratio of H to F increases (i.e. CH₃F>CH₂F₂>CHF₃). See,e.g., Chen et. al. (Microelectronic Engineering 86, (2009)).

However, reports have shown that the preferred etch gas, CH₃F, mayimplant carbon into the silicon substrate during nitride spacer etching,requiring an additional processing step (HBr/O₂ or N₂/H₂ plasma) toremove C—Si bonds prior to the silicon epitaxy step. Blanc et al.,Journal of Vacuum Science & Technology B 32 (2) (March/April 2014).

Additionally, as further explained in more detail in the presentapplication, while H rich etch molecules may selectively etch siliconnitride, some of these etch gases have characteristics which are notdesirable for application in gate spacer layer etching.

A need remains for SiN etching processes suitable for gate spacer layeretching without detriment to the processes that follow.

NOTATION AND NOMENCLATURE

Certain abbreviations, symbols, and terms are used throughout thefollowing description and claims, and include:

As used herein, the indefinite article “a” or “an” means one or more.

As used herein, the terms “approximately” or “about” mean±10% of thevalue stated.

As used herein, the term “etch” or “etching” refers to a plasma etchprocess (i.e., a dry etch process) in which ion bombardment acceleratesthe chemical reaction in the vertical direction so that verticalsidewalls are formed along the edges of the masked features at rightangles to the substrate (Manos and Flamm, Plasma Etching AnIntroduction, Academic Press, Inc. 1989 pp. 12-13).

The term “selectivity” means the ratio of the etch rate of one materialto the etch rate of another material. The term “selective etch” or“selectively etch” means to etch one material more than anothermaterial, or in other words to have a greater or less than 1:1 etchselectivity between two materials. Infinite selectivity occurs when theetch gas etches one material but does not etch the other.

As used herein, the abbreviation “FinFET” refers to a fin structuredfield-effect transistor; the abbreviation “FD-SOI” refers tofully-depleted silicon-on-insulator; and the abbreviation “3D” refers to3 dimensional or vertical gate structures.

The standard abbreviations of the elements from the periodic table ofelements are used herein. It should be understood that elements may bereferred to by these abbreviations (e.g., N refers to nitrogen, Sirefers to silicon, H refers to hydrogen, etc.).

Please note that the silicon nitride, silicon oxide, and silicon layers,such as SiN, SiO and poly-Si, are listed throughout the specificationand claims without reference to their proper stoichoimetry. The siliconlayers may include pure silicon (Si) layers, such as crystalline Si,polysilicon (poly-Si or polycrystalline Si), or amorphous silicon. TheSiN layers may optionally include other atoms, such as oxygen, carbon,or boron (i.e., Si_(a)N_(b), SiO_(a)N_(b), SiC_(c)N_(b),SiO_(a)C_(c)N_(b), or SiB_(d)C_(c)N_(b), wherein each of a, b, c, and dindependently range from approximately 0.05 to approximately 0.95). Thesilicon oxide (Si_(n)O_(m)) layers may have stoichiometry wherein m andn inclusively range from 1 to 6. Preferably silicon oxide isSi_(n)O_(m), where n ranges from 0.5 to 1.5 and m ranges from 1.5 to3.5. More preferably, silicon oxide is SiO₂ or SiO₃. The silicon oxidelayer could also be a dielectric material, such as organic based orsilicon oxide based low-k dielectric materials such as the Black DiamondII or III material by Applied Materials, Inc. Any of thesilicon-containing layers (i.e., Si, SiN, SiO) may also include dopants,such as B, C, P, As and/or Ge.

SUMMARY

Silicon nitride plasma etching processes for etching SiN layers fromsubstrates are disclosed. A SiN roughness layer between approximately 0nm and 10 nm thick is generated during the silicon nitride plasmaetching process by simultaneously introducing an oxidizer at apredetermined flow rate and an etch gas into a plasma reaction chambercontaining the substrate. The etch gas has the formula C_(x)H_(y)F_(z),wherein x is 2-5, z is 1 or 2, 2x+2=y+z, and a fluorine atom is locatedon a terminal carbon atom of the etch gas. The disclosed processes mayinclude one or more of the following aspects:

-   -   producing infinite SiN to substrate selectivity;    -   determining the predetermined flow rate by (a) plotting a SiN        etch rate for the etch gas on a Y axis versus oxidizer flow rate        on a X axis; (b) plotting a substrate etch rate for the etch gas        on a Y axis versus oxidizer flow rate on a X axis; and (c)        selecting the predetermined flow rate when the SiN etch rate is        positive, the substrate etch rate is equal to or less than 0        nm/minute, and the SiN etch rate remains approximately the same        with increasing oxidant flow rate;    -   determining the predetermined flow rate by optical or surface        topography analysis of a substrate having a partial SiN layer        deposited thereon to determine an oxidizer flow rate that (a)        etches the SiN layer, (b) does not etch the substrate, and (c)        does not generate a SiN roughness layer greater than 10 nm        thick, wherein the partial SiN layer does not cover the entire        substrate;    -   optically analyzing the substrate with an ellipsometer;    -   optically analyzing the substrate with an electron microscope;    -   optically analyzing the substrate with a scanning electron        microscope;    -   optically analyzing the substrate with a transmission electron        microscope;    -   analyzing the surface topography of a substrate with an atomic        force microscope;    -   ending the process by measuring a time when the substrate below        the SiN layer begins to accumulate polymer deposits;    -   the etch gas selectively reacting with SiN to form volatile        by-products;    -   removing volatile by-products from the plasma reaction chamber;    -   analyzing the volatile by-products removed from the plasma        reaction chamber;    -   ending the process when generation of volatile by-products        ceases;    -   z being 1;    -   the etch gas being selected from the group consisting of        fluoroethane, 1-fluoropropane; 1-fluorobutane; and        1-fluoropentane;    -   the etch gas being fluoroethane;    -   the etch gas being 1-fluoropropane;    -   the etch gas being 1-fluorobutane;    -   the etch gas being 1-fluoropentane;    -   z being 2;    -   x being 3;    -   the etch gas being 1,1-difluoropropane;    -   the etch gas being 1,2-difluoropropane;    -   the etch gas being 1,1-difluorobutane;    -   the etch gas being 1,2-difluorobutane;    -   the etch gas being 1,1-difluoropentane;    -   the etch gas being 1,2-difluoropentane;    -   the oxidizer being selected from the group consisting of O₂, CO,        CO₂, NO, N₂O, NO₂, SO₂, O₃, and combinations thereof; and    -   the oxidizer being O₂.    -   mixing the etch gas and the oxidizer prior to introduction to        the chamber;    -   introducing the etch gas into the chamber separately from the        oxidizer;    -   introducing the oxidizer continuously to the chamber and        introducing the etch gas to the chamber in pulses;    -   introducing into the chamber approximately 5% v/v to        approximately 100% v/v of oxidizer;    -   introducing an inert gas into the plasma reaction chamber;    -   the inert gas being selected from the group consisting of He,        Ar, Xe, Kr, and Ne;    -   the inert gas being Ar;    -   mixing the etch gas and the inert gas prior to introduction into        the chamber to produce a mixture;    -   introducing the etch gas into the chamber separately from the        inert gas;    -   introducing the inert gas being continuously into the chamber        and introducing the etch gas into the chamber in pulses;    -   the inert gas comprising approximately 25% v/v to approximately        95% v/v of a total volume of etch gas, oxidizer and inert gas        introduced into the plasma reaction chamber;    -   the silicon nitride layer being selected from the group        consisting of silicon nitride, silicon oxynitride, silicon        oxycarbonitride, silicon carbonitride, silicon borocarbonitride,        and combinations thereof;    -   the substrate being selected from the group consisting of        silicon, polysilicon, silicon oxide, SiGe, Ge, GaAs, InGaAs,        InP, InAs, or combinations thereof;    -   the substrate being polysilicon;    -   the substrate being silicon dioxide;    -   selectively etching the silicon nitride layer from a silicon        oxide layer;    -   selectively etching the silicon nitride layer from a polysilicon        layer;    -   the silicon nitride plasma etching processes etching a gate        spacer layer;    -   the etched gate spacer layer not requiring subsequent HBr/O₂ or        N₂/H₂ plasma treatment;    -   improving selectivity by introducing a depositing gas into the        plasma reaction chamber;    -   the depositing gas being selected from the group consisting of        CH₄, CFH₃, CH₂F₂, C₂H₆, C₃H₈, and C₄H₁₀;    -   mixing the etch gas and the depositing gas prior to introduction        to the chamber;    -   introducing the depositing gas into the chamber separately from        the etch gas;    -   introducing approximately 1% v/v to approximately 99.9% v/v of        the depositing gas into the chamber;    -   activating the plasma by a RF power ranging from approximately        25 W to approximately 10,000 W;    -   the chamber having a pressure ranging from approximately 1 mTorr        to approximately 10 Torr; more preferably the pressure ranging        from 50 mtorr to 200 mtorr;    -   introducing the etch gas to the chamber at a flow rate ranging        from approximately 0.1 sccm to approximately 1 slm;    -   maintaining the substrate at a temperature ranging from        approximately −196° C. to approximately 500° C.;    -   maintaining the substrate at a temperature ranging from        approximately −120° C. to approximately 300° C.;    -   maintaining the substrate at a temperature ranging from        approximately −10° C. to approximately 40° C.;    -   measuring the activated etch gas by Quadropole mass        spectrometer, optical emission spectrometer, FTIR, or other        radical/ion measurement tool; and    -   generating the plasma being by applying RF power.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1A is a diagram showing the exemplary layer of silicon nitride on asubstrate;

FIG. 1B is a diagram showing the exemplary layer of silicon nitride on asubstrate after the disclosed etch process;

FIG. 2 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor fluoromethane (CH₃F);

FIG. 3 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor fluoroethane (C₂H₅F);

FIG. 4 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor 1,1,2-trifluoroethane (C₂H₃F₃);

FIG. 5 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor 1-fluoropropane (C₃H₇F);

FIG. 6 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor 1,1-difluoropropane (C₃H₆F₂);

FIG. 7 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor 1,1,1-trifluoropropane (C₃H₅F₃);

FIG. 8 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor 1,2-difluoropropane;

FIG. 9 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor 1,3-difluoropropane;

FIG. 10 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor 2,2-difluoropropane;

FIG. 11 is a scanning electron micrograph (SEM) of the results of a 2minute SiN etch using 20 sccm CH₃F, 50 sccm Ar, 6 sccm O₂, at a pressureof 5 Pa and 200 W RF power;

FIG. 12 is a graph of SiN roughness layer thickness versus O₂ flow forthe same CH₃F etch processes in FIG. 2;

FIG. 13 is a scanning electron micrograph (SEM) of the results of a 2minute SiN etch using 20 sccm C₂H₃F₃, 50 sccm Ar, 12.6 sccm O₂, at apressure of 5 Pa and 200 W RF power;

FIG. 14 is a graph of SiN roughness layer thickness versus O₂ flow forthe same C₂H₃F₃ etch processes in FIG. 4;

FIG. 15 is a scanning electron micrograph (SEM) of the results of a 2minute SiN etch using 20 sccm C₃H₇F, 50 sccm Ar, 23.4 sccm O₂, at apressure of 5 Pa and 200 W RF power;

FIG. 16 is a graph of SiN roughness layer thickness versus O₂ flow forthe same CH₃F etch processes in FIG. 5;

FIG. 17 is a mass spectrometry (MS) graph plotting the volume of speciesfractions produced by CH₃F versus energy;

FIG. 18 is a MS graph plotting the volume of species fractions producedby fluoroethane versus energy;

FIG. 19 is a MS graph plotting the volume of species fractions producedby 1,1,2-trifluoroethane versus energy;

FIG. 20 is a MS graph plotting the volume of species fractions producedby 1-fluoropropane versus energy;

FIG. 21 is a MS graph plotting the volume of species fractions producedby 1,1-difluoropropane versus energy;

FIG. 22 is a MS graph plotting the volume of species fractions producedby 2,2-difluoropropane versus energy;

FIG. 23 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor 1-fluorobutane (C₄H₉F);

FIG. 24 is is an XPS graph providing the depth profile of a polymerdeposited layer from 1-F—C₃H₇ on a poly-Si substrate;

FIG. 25 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor fluoromethane (CH₃F);

FIG. 26 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor fluoroethane (C₄H₉F);

FIG. 27 is a graph of SiN, SiO₂, and poly-Si etch rates versus O₂ flowfor 1-fluoropropane (C₃H₇F);

FIG. 28 is a graph of SiN roughness layer thickness versus O₂ flow forthe same CH₃F etch processes in FIG. 25;

FIG. 29 is a graph of SiN roughness layer thickness versus O₂ flow forthe same C₂H₅F etch processes in FIG. 26;

FIG. 30 is a graph of SiN roughness layer thickness versus O₂ flow forthe same 1-F—C₃H₇ etch processes in FIG. 27;

FIG. 31 is a graph of poly-Si loss versus O₂ flow for a 200 W CH₃Fplasma;

FIG. 32 is a graph of poly-Si loss versus O₂ flow for a 50 W CH₃Fplasma;

FIG. 33 is a graph of poly-Si loss versus O2 flow for a 50 W C2H5Fplasma;

FIG. 34 is a graph of poly-Si loss versus O₂ flow for a 50 W 1-F—C₃H₇plasma; and

FIG. 35 is a flowchart for the disclosed etching process.

DESCRIPTION OF PREFERRED EMBODIMENTS

Etch gases for improved silicon nitride plasma etching methods aredisclosed. The disclosed etch gases fully etch the SiN layers from asubstrate without generating a large SiN roughness layer. The disclosedetch gases also provide improved selectivity to substrate materials.

The disclosed plasma etch gases provide improved selectivity between thesilicon nitride layers and substrate materials. Additionally, thedisclosed plasma etch gases etch the silicon nitride layers whilemaintaining very low surface roughness. The disclosed plasma etch gasesalso provide protection from damage to substrate materials by formationof a polymer layer.

The disclosed plasma etch gases have the formula C_(x)H_(y)F_(z),wherein x is 2-5, z is 1 or 2, 2x+2=y+z, and a fluorine atom is locatedon a terminal carbon atom of the etch gas. Exemplary etch gases includefluoroethane; 1-fluoropropane; 1,1-difluoropropane; 1,2-difluoropropane,1-fluorobutane, 1,1-fluorobutane, 1,2-difluorobutane, 1-fluoropentane,1,1-difluoropentane, 1,2-difluoropentane, and combinations thereof. Forexample, the plasma etch gas may include a combination of 1-fluoroethanewith 1-fluoropropane or 1-fluoropropane with 1-fluorobutane. Thesecompounds are commercially available. Even though some of thesecompounds have the same molecular formula (i.e., C₃H₆F₂), each isomerhas different bonding structure and therefore may produce differentfragmentation in the plasma, resulting in different etching ordeposition species.

As shown in the examples that follow, some of the prior art etch gasesgenerate large SiN roughness layers, resulting in an uneven etchprocess. The rough SiN layer may permit exposure of the underlyingsubstrate to more physical plasma damage and/or chemical damage. Forexample, the uneven SiN layer may permit penetration of carbon andoxygen into the substrate, which may form parasitic carbide or oxidelayers, which may prevent epitaxial silicon growth in the nextsource/drain contact processing step. Additionally, the uneven SiN layerresults in formation of an uneven polymer protection layer during theetch process. The rough SiN layers may also require longer SiN etchprocessing time, potentially exposing the underlying layer to moredamage from the plasma process. Any chemical or physical damage to thesubstrate will degrade device performance.

Applicants have discovered that the fluorine atom located on theterminal carbon atom of the etch gas provides a larger process rangewith infinite SiN:substrate selectivity and low SiN roughness during theSiN etch process. Applicants believe that etch gases with the F on theterminal C produces hydrogen rich C₂ and/or C₃ and/or C₄ fragments inthe plasma which are beneficial because these fragments produce lessdamage to the underlying substrate when compared to F rich fragments.More particularly, because plasma treatment of these etch gases producefragments containing longer alkyl chains than the commercially used etchgas, CH₃F, Applicants believe protective polymer layers will bedeposited on the substrate rather than being implanted as carbon intothe substrate.

The disclosed plasma etch gases are provided at between approximately99.9% by volume and approximately 100.0% by volume purity, preferablybetween approximately 99.99% by volume and approximately 100.00% byvolume purity, and more preferably between approximately 99.999% byvolume and approximately 100.000% by volume purity. The disclosed etchgases contain between approximately 0.0% by volume and approximately0.1% by volume trace gas impurities with between approximately 0 ppm byvolume to approximately 150 ppm by volume of nitrogen-containing andoxygen-containing gases, such as N₂ and/or H₂O and/or CO₂ and/or CO,and/or SO₂ contained in said trace gaseous impurities. Preferably, thewater content in the plasma etch gas is between approximately 0 ppm byweight and approximately 20 ppm by weight. The purified product may beproduced by distillation and/or passing the gas or liquid through asuitable adsorbent, such as a 4A molecular sieve.

In one alternative the disclosed plasma etch gas contains betweenapproximately 0% by volume and approximately 5% by volume, preferablybetween approximately 0% by volume and approximately 1% by volume, morepreferably between approximately 0.0% by volume and approximately 0.1%by volume, and even more preferably between approximately 0.00% byvolume and approximately 0.01% by volume of any of its isomers. Thisalternative may provide better process repeatability. This alternativemay be produced by distillation of the gas or liquid. Alternatively, thedisclosed plasma etch gas may contain between approximately 5% by volumeand approximately 50% by volume of one or more of its isomers,particularly when the isomer mixture provides improved processparameters or isolation of the target isomer is too difficult orexpensive. For example, a mixture of isomers may reduce the need for twoor more gas lines to the plasma reactor. One exemplary mixture maycombine 50% by volume 1,1-C₃H₆F₂ with 50% by volume 1,2-C₃H₆F₂ or 90%1,1-C₄H₈F₂ with 10% 1,2-C₄H₈F₂.

The disclosed compounds are suitable for plasma etching silicon nitridelayers used as gate spacer layers, because they induce little to nodamage on underlying substrate materials. In order to achieve thoseproperties, the hydrogen rich etch gas may deposit an etch-resistantpolymer layer during etching and help reduce the direct impact of theoxygen and fluorine radicals during the etching process. Preferably, thehydrogen rich etch gas is both suitably volatile and stable during theetching process for delivery into the reactor/chamber.

The disclosed etch gases plasma etch silicon nitride layers on asubstrate. The disclosed plasma etching method may be useful in themanufacture of semiconductor devices such as FinFET or FD-SOI. The otherareas of applications include its use in different front end of the line(FEOL).

The disclosed plasma etch gases minimize formation of a SiN roughnesslayer on the substrate during the plasma etch process. Moreparticularly, by simultaneously introducing the disclosed etch gases andan oxidizer at a predetermined flow rate into a plasma reaction chambercontaining the substrate, the disclosed methods generate a SiN roughnesslayer between approximately 0 nm and 10 nm thick on the substrate.

The plasma reaction chamber may be any enclosure or chamber within adevice in which etching methods take place such as, and withoutlimitation, Reactive Ion Etching (RIE), Dual Capacitively Coupled Plasmawith single or multiple frequency RF sources, Inductively Coupled Plasma(ICP), Remote Plasma, Pulsed Plasma, or Microwave Plasma reactors, orother types of etching systems capable of selectively removing a portionof the silicon nitride layer or generating active species. One ofordinary skill in the art will recognize that the different plasmareaction chamber designs provide different electron temperature control.Suitable commercially available plasma reaction chambers include but arenot limited to the Applied Materials magnetically enhanced reactive ionetcher sold under the trademark eMAX™ or the Lam Research Dual CCPreactive ion etcher Dielectric etch product family sold under thetrademark 2300® Flex™.

The plasma reaction chamber may contain one or more than one substrate.A substrate is generally defined as the material on which a process isconducted. For example, the plasma reaction chamber may contain from 1to 200 silicon wafers having from 25.4 mm to 450 mm diameters. The oneor more substrates may be any suitable substrate used in semiconductor,photovoltaic, flat panel or LCD-TFT device manufacturing. Typically thesubstrate will be a patterned substrate having multiple layers thereon.Examples of suitable layers include without limitation silicon (such asamorphous silicon, polysilicon (aka Poly-Si), crystalline silicon, anyof which may further be p-doped or n-doped), silica, silicon nitride,silicon oxide, silicon oxynitride, tungsten, titanium nitride, tantalumnitride, mask materials such as amorphous carbon, antireflectivecoatings, photoresist materials, or combinations thereof. Additionally,layers comprising tungsten or noble metals (e.g. platinum, palladium,rhodium or gold) may be used. One of ordinary skill in the art willrecognize that the terms “film” or “layer” used herein refer to athickness of some material laid on or spread over a surface and that thesurface may be a trench or a line. Throughout the specification andclaims, the wafer and any associated layers thereon are referred to assubstrates.

For example, the substrate may form a silicon nitride gate spacersimilar to the structure shown in FIG. 1A. In FIG. 1A, a silicon nitridelayer 400 is deposited on a gate structure 100 and silicon layer 200,all located on a buried oxide silicon wafer 300. The silicon nitridelayer 400 may include only Si and N atoms in a 1:1 ratio. Alternatively,the silicon nitride layer 400 may include only Si and N atoms in a 3:4ratio. In another alternative, the SiN layer 400 may include only Si andN atoms in a ratio of 1 Si to 0.05 to 0.95N. In yet another alternative,the SiN layer 400 may include other atoms, such as oxygen, carbon, orboron, to form SiO_(a)N_(b), SiC_(c)N_(b), SiO_(a)C_(c)N_(b), orSiB_(d)C_(c)N_(b), wherein each of a, b, c, and d independently rangefrom approximately 0.05 to approximately 0.95. The silicon nitride layer400 may be between approximately 2 and approximately 50 nm thick. Thegate structure 100 may include any of the standard layers used to formgate structures, such as dielectric and electrode layers. In oneexample, the top of the gate structure 100 in direct contact with thesilicon nitride layer 400 is a silicon oxide dielectric layer.

A schematic of the structure after exposure to the disclosed etchprocess is provided in FIG. 1B. The disclosed etch process isanisotropic and selective to the silicon nitride layer 400. Therefore,the remaining vertical silicon nitride layer 410 may be used as a gatespacer. The gate structure 100, the silicon layer 200, and the buriedoxide layer 300 remain unaltered due to the infinite silicon nitride tosubstrate selectivity of the processes disclosed herein.

The etch gas is introduced into the chamber containing the substrate andsilicon nitride layers. The etch gas may be introduced to the chamber ata flow rate ranging from approximately 0.1 sccm to approximately 1 slm.For example, for a 200 mm wafer size, the etch gas may be introduced tothe chamber at a flow rate ranging from approximately 5 sccm toapproximately 50 sccm. Alternatively, for a 450 mm wafer size, the etchgas may be introduced to the chamber at a flow rate ranging fromapproximately 25 sccm to approximately 250 sccm. One of ordinary skillin the art will recognize that the flow rate will vary from tool totool.

Some of the disclosed etch gases may be liquids at standard temperatureand pressure. One of ordinary skill in the art will recognize how toconvert the liquid to gas form for introduction into the chamber. Forexample, the liquid may be vaporized through direct vaporization,distillation, direct liquid injection, or by bubbling. The liquid may befed to a vaporizer where it is vaporized before it is introduced intothe reactor. Alternatively, the liquid may be vaporized by passing acarrier gas into a container containing the compound or by bubbling thecarrier gas into the compound. The carrier gas may include, but is notlimited to, Ar, He, N₂, and mixtures thereof. Bubbling with a carriergas may also remove any dissolved oxygen present in the neat or blendedcompound solution. The carrier gas and compound are then introduced intothe reactor in gaseous form.

An oxidizer, such as O₂, O₃, CO, CO₂, NO, N₂O, NO₂, SO₂, andcombinations thereof, is simultaneously introduced at a predeterminedflow rate into the plasma reaction chamber with the etch gas. The etchgas and the oxidizer may be mixed together prior to introduction intothe chamber. Alternatively, the oxidizer may be introduced continuouslyinto the chamber and the etch gas introduced into the chamber in pulses.The oxidizer may comprise between approximately 5% by volume toapproximately 100% by volume of the mixture introduced into the chamber(with 100% by volume representing introduction of pure oxidizer for thecontinuous introduction alternative).

Applicants have surprisingly discovered that varying the oxidant flowrate in the nitride etch process results in two different phenomena. Theoxidant flow rate may be optimized to produce infinite selectivelybetween SiN and the underlying substrate and/or surrounding layers. Theoxidant flow rate may also be optimized to minimize roughness in the SiNlayer. As illustrated in the examples that follow, the oxidant flowrates for these two phenomena do not overlap for all SiN etch gases.Applicants believe that having one F on a terminal carbon of the etchgas permits wider oxidant flow rate windows for both phenomena,providing overlap of the two, and improved SiN etch processes.

The disclosed processes selectively etch the SiN layer without etchingthe underlying substrate and/or any other exposed non-SiN layers on thesubstrate. Infinite selectivity occurs when the etch rate of SiN is apositive number and the etch rate of the substrate occurs at anegligible rate that is approximately 0 nm/min or any negative etch rate(i.e., deposition of polymer). The present application makes nodistinction between an etch rate of 0 nm/min and deposition of polymer.Variations in film thickness measurement techniques, instrumentvariation, topographical variations in the substrate, and naturalfluctuations such as machine noise or uncontrollable variations mayperiodically lead to some measured substrate etch rates greater than 0nm/min. For the purpose of this disclose, a substrate etch rate of 0 or“without etching” means a measured etch rate equal to or less than 0nm/min or a measured etch rate greater than 0 nm/min but less than theprecision of the film thickness measurement technique.

One of ordinary skill in the art will recognize that a process providinginfinite selectivity is important because it permits removal of the SiNlayer without affecting the surrounding or underlying layers. Many SiNetch processes are performed longer than necessary to ensure completeremoval of the SiN layer. For example, a 10 nm SiN layer may be etchedfor the amount of time necessary to remove a 13 nm SiN layer just toensure that no SiN remains on the substrate. Obviously, as a result, theunderlying substrate is subject to the potential for more damage in sucha process.

SiN etching with some prior art SiN etch gases produces a rough SiNlayer, similar to those shown in FIGS. 11 and 13 of Example 5. The roughSiN layer may expose the underlying substrate to plasma damage and/orchemical damage. Additionally, the uneven SiN layer results in an unevenpolymer protection layer during the etch process. The rough SiN layersmay also require longer SiN etch processing times in order to completelyremove the SiN layer, potentially exposing the underlying layer to moredamage during the plasma process.

One of ordinary skill in the art will recognize that the predeterminedoxidant flow rate will vary amongst plasma reaction chambers. Thepredetermined flow rate may be calculated by optical or surfacetopography analysis of a substrate having a partial SiN layer depositedthereon during the etch process to determine an oxidizer flow rate that(a) etches the SiN layer, (b) does not etch the substrate, and (c) doesnot generate a SiN roughness layer greater than 10 nm thick. Theanalysis may be performed by any device having sufficient resolution toview nanometer sized structures in the 0-10 nm range. Exemplary surfacetopography analysis devices include, but are not limited to, atomicforce microscopes. Exemplary optical analysis devices include, but arenot limited to, variable angle spectroscopic ellipsometers, scanningelectron microscopes, and transmission electron microscopes. The partialSiN layer may be deposited on and/or surrounded by a substrate of adiffering material. For example, the SiN layer may be deposited on apolysilicon layer and surrounded by a silicon dioxide layer. The etchprocess may be repeated at varying oxidant flow rates until an oxidantflow rate is determined that successfully etches the SiN layer to lowroughness and fails to etch the underlying polysilicon and/orsurrounding silicon dioxide layers.

Alternatively, etch rates may be plotted to determine when the oxidantflow rates producing infinite selectivity overlap those producing lowSiN roughness. More particularly, the range of oxidant flow ratesproducing infinite selectivity may be determined by plotting the SiNetch rate on a Y axis versus oxidizer flow rate on a X axis and plottingthe substrate etch rate for the etch gas on a Y axis versus oxidizerflow rate on a X axis. One of ordinary skill in the art will recognizethat infinite selectivity occurs in the range of oxidant flow rates inwhich SiN has a positive etch rate and the substrate has no etch rate(i.e., 0 nm/minute or any deposition of polymer—the deposition ofpolymer is sometimes shown as a negative etch rate). As shown in theExamples that follow, experimental results indicate that low SiNroughness (i.e., etching that leaves a SiN roughness layer betweenapproximately 0 nm and approximately 10 nm thick) correlates to the SiNetch rate reaching a plateau on a plot of etch rate on the Y axis versusoxidizer flow rate on the X axis [i.e., when the SiN etch rate remainsapproximately the same (±10%, preferably ±5%) with increasing oxidantflow rate]. The predetermined oxidant flow rate may be selected frompoints on the graph having a substrate etch rate equal to or less than 0nm/minute and a slope of the SiN etch rate of approximately 0.

An inert gas is also introduced into the reactor in order to sustain theplasma. The inert gas may be He, Ar, Xe, Kr, Ne, or combinationsthereof. The etch gas and the inert gas may be mixed prior tointroduction to the chamber, with the inert gas comprising betweenapproximately 50% by volume and approximately 95% by volume of theresulting mixture. Alternatively, the inert gas may be introduced to thechamber continuously while the etch gas is introduced to the chamber inpulses.

The etch gas, oxidizer and inert gas are activated by plasma. The plasmadecomposes the etch gas and oxidizer into radical form. The plasma maybe generated by applying RF or DC power. The plasma may be generatedwith a RF power ranging from about 25 W to about 10,000 W. The plasmamay be generated or present within the reactor itself. Alternatively, aremotely located plasma system may be used. The plasma may be generatedin Dual CCP or ICP mode with RF applied at both electrodes. RF frequencyof plasma may range from 200 KHz to 1 GHz. Different RF sources atdifferent frequency can be coupled and applied at same electrode. PlasmaRF pulsing may be further used to control molecule fragmentation andreaction at substrate. One of skill in the art will recognize methodsand apparatus suitable for such plasma treatment.

Quadrupole mass spectrometer, optical emission spectrometer, FTIR, orother radical/ion measurement tools may measure the activated etch gasto determine the types and numbers of species produced. If necessary,the flow rate of the etch gas, oxygen, and/or the inert gas may beadjusted to increase or decrease the number of radical species produced.

The disclosed etch gases may be mixed with other gases either prior tointroduction into the reaction chamber or inside the reaction chamber.Preferably, the gases may be mixed prior to introduction to the chamberin order to provide a uniform concentration of the entering gas. Inanother alternative, the etch gas may be introduced into the chamberindependently of the other gases such as when two or more of the gasesreact.

Other exemplary gases with which the etch gases may be mixed includeadditional etch gases, such as CF₄, CH₃F, CH₂F₂, and CHF₃. The two gasesmay be mixed prior to introduction to the chamber. The etch gas maycomprise between approximately 1% by volume to approximately 99.9% byvolume of the mixture introduced into the chamber.

Other exemplary gases with which the etch gases may be mixed includeadditional deposition gases, such as CH₄, C₂H₆, C₃H₈, and NH₃. The etchgas and the deposition gas may be mixed prior to introduction to thechamber. The etch gas may comprise between approximately 1% by volume toapproximately 99.9% by volume of the mixture introduced into thechamber.

The silicon nitride layers and the activated etch gas react to formvolatile species that are removed from the reactor. At the predeterminedoxidant flow rates, the silicon oxide and silicon are less reactive, andpreferably non-reactive, to the etch gas.

The temperature and the pressure within the reactor are held atconditions suitable for the silicon nitride layer to react with theactivated etch gas. For instance, the pressure in the reactor may beheld between approximately 0.1 mTorr and approximately 1000 Torr,preferably between approximately 1 mTorr and approximately 10 Torr, morepreferably between approximately 50 mTorr and approximately 500 mTorr,and more preferably between approximately 50 mTorr and approximately 200mTorr, as required per the etching parameters. Likewise, the substratetemperature in the reactor may range between about approximately −196°C. to approximately 500° C., preferably between −120° C. toapproximately 300° C., and more preferably between −10° C. toapproximately 40° C. Chamber wall temperatures may range fromapproximately −196° C. to approximately 300° C. depending on the processrequirements.

The reactions between the silicon nitride layer and the plasma activatedetch gas results in removal of the silicon nitride layer from thesubstrate. Atoms of oxygen and/or carbon and/or boron may also bepresent in the silicon nitride layer. The removal is due to a physicalsputtering of the silicon nitride layer from plasma ions (accelerated bythe plasma) and/or by chemical reaction of plasma species to convert SiNto volatile by-products, such as SiF_(x), wherein x ranges from 1-4, andHCN. The physically sputtered materials and/or volatile by-products maybe removed from the reaction chamber via vacuum.

The plasma activated etch gas preferably exhibits high selectivitytoward the substrate and etches SiN anisotropically which is importantfor gate spacer applications. The plasma activated etch gas preferablyselectively etches SiN over the underlying substrate material, such assilicon, silicon oxide, poly-silicon, SiGe, GaAs, InGaAs, or InP.

The silicon nitride etching process may be stopped when the underlyingsubstrate begins to accumulate polymer deposition. Alternatively, theconcentration of volatile by-products in the chamber effluent may beanalyzed and the etch process stopped when generation of the volatileby-products ceases.

FIG. 35 is a flowchart for the disclosed etching process. At step 102, asubstrate is etched with an infinite SiN to substrate selectivity and aSiN roughness layer of between approximately 0 nm and approximately 10nm thickness is generated on a SiN layer of the substrate.Simultaneously, at step 104, an oxidizer is introduced into a plasmareaction chamber containing the substrate at a flow rate. Then an etchgas is introduced into the plasma reaction chamber, wherein the etch gasselectively reacts with SiN to form volatile by-products at step 106.The etch gas selectively reacts with SiN to form volatile by-products inthe plasma reaction chamber. The volatile by-products is then removedfrom the plasma reaction chamber at step 108. At step 110, the etchingprocess may be ended by analyzing the volatile by-products removed fromthe plasma reaction chamber to determine when generation of the volatileby-products ceases.

The disclosed etch processes using the etch gases to etch siliconnitride layers maintains the physical and chemical integrity of theunderlying substrate material. The underlying substrate material mayhave a protecting polymer layer as a result of the etch process. Theprotecting polymer layer may act as a barrier to etching or damage ofthe underlying substrate material from plasma species.

As shown in the examples that follow, the disclosed etch gases enjoy awide oxidant flow rate process window that provides infiniteSiN:substrate selectivity and generates small SiN roughness layersduring the etch process. Applicants believe that generating smaller SiNroughness layers during the etch process better protects the underlyingsubstrate from plasma damage and carbon implantation, which may preventthe necessity of additional processing steps, such as HBr/O₂ or N₂/Hplasma, prior to the silicon epitaxy step.

In one non-limiting exemplary plasma etch process, the vapor of C₂H₅F isintroduced into a 200 mm Reactive Ion Etch plasma etch tool using acontrolled gas flow device. The pressure of the plasma etch tool is setat approximately 30 mTorr. No gas source heating is necessary, as thevapor pressure of this compound is approximately 5600 torr at roomtemperature. The electrode RF power is fixed at 200 W. The plasma etchtool includes a chamber containing a substrate having silicon nitridelayers thereon. Argon is independently introduced into the chamber at a50 sccm flow rate. C₂H₅F is independently introduced into the chamber at20 sccm. O₂ is independently introduced into the chamber at 0-30 sccm todetermine optimum etching conditions.

EXAMPLES

The following non-limiting examples are provided to further illustrateembodiments of the invention. However, the examples are not intended tobe all inclusive and are not intended to limit the scope of theinventions described herein.

The following testing was performed using a SAMCO10-NR reactive ionetcher (RIE). Samples were cleaved into 1×1 cm2 from 200 mm wafers withblanket films of SiN, polysilicon (poly-Si), and TEOS. The blanket filmthickness was measured on a variable angle spectroscopic ellipsometer.Film thickness was approximately 270 nm for SiN, 300 nm for poly-Si, and2000 nm for TEOS. The samples were loaded into a 200 mm plasma chamberand placed on top of the RF powered electrode. No heating or cooling wasapplied to the substrate. The chamber was evacuated to ˜10⁻² Pa. The Ar,O₂, and etch gas were then simultaneously introduced into the chamber atpredetermined flow rates through separate gas lines. The gases wereallowed to equilibrate in the plasma chamber at a pressure of 5 Pa. Theplasma was then turned on using an RF power ranging from 50 W to 200 Wand the process duration was 2 minutes. The process time was chosen sothat a portion of each film was removed and could be measured using theabove mentioned ellipsometer.

Example 1

Etch data using C₂H₅F and C₂H₃F₃ gases were analyzed with respect toCH₃F etch data. All etch data was collected under 200 W plasma RF power.The data serves as a comparison for molecules with H:F>1 versus H:F≤1. Agraph of the SiN, SiO₂, and poly-Si etch rates versus O₂ flow (instandard cubic centimeters per minute (sccm)) for fluoromethane (CH₃F)is shown in FIG. 2. A graph of the SiN, SiO₂ and poly-Si etch ratesversus O₂ flow for fluoroethane (C₂H₅F) is shown in FIG. 3. A graph ofthe SiN, SiO₂, and poly-Si etch rates versus O₂ flow for1,1,2-trifluoroethane (C₂H₃F₃) is shown in FIG. 4. In FIGS. 2-10, asillustrated by the circle and arrow, the SiN etch rate (in nm/min) isprovided on the left Y axis and the SiO₂ and Poly-Si etch rates (innm/min) are provided on the right Y axis. Additionally, in FIGS. 2-10,the SiO₂ etch rate is labeled as TEOS because the SiO₂ film wasdeposited from the tetraethyl orthosilicate precursor.

The higher H:F ratio for C₂H₅F yields a larger infinite selectivityrange (ISR) relative to CH₃F. The ISR is defined as the range of O₂ flow(in sccm) where infinite SiN to poly-Si selectivity is obtained.Infinite SiN to poly-Si selectivity occurs when SiN etches and poly-Sidoes not etch or SiN etches and poly-Si accumulates polymer deposits.There is no distinction between an etch rate of 0 nm/min and depositionof polymer for the data presented in FIGS. 2-10. In FIG. 2, the ISR forCH₃F ranges from approximately 5.4 sccm O₂ through approximately 6.6sccm O₂. In FIG. 3, the ISR for C₂H₅F for ranges from approximately 7.2sccm O₂ through approximately 10.2 sccm O₂. The SiN etch rate is alsolarger for C₂H₅F in the infinite selectivity range.

C₂H₃F₃ has a lower H:F ratio and lower infinite selectivity rangerelative to CH₃F. In FIG. 4, the ISR for C₂H₃F₃ ranges fromapproximately 12.0 sccm O₂ through approximately 12.6 sccm O₂.

The calculated ISR values are presented in Table 1 in Example 8.

Example 2

Etch data using C₃H₇F (1-fluoropropane), C₃H₆F₂ (1,1-difluoropropane),and C₃H₅F₃ (1,1,1-trifluoropropane) gases were analyzed. All etch datawas collected under 200 W plasma RF power. The data serves as acomparison of F content on a terminal carbon in C3 molecules. The graphof the SiN, SiO₂, and poly-Si etch rates versus O₂ flow (in sccm) forfluoropropane (C₃H₇F) is shown in FIG. 5. The graph of the SiN, SiO₂ andpoly-Si etch rates versus O₂ flow (in sccm) for 1,1-difluoropropane(C₃H₆F₂) is shown in FIG. 6. The graph of the SiN, SiO₂ and poly-Si etchrates versus O₂ flow (in sccm) for 1,1,1-trifluoropropane (C₃H₅F₃) isshown in FIG. 7. The Process Window (defined as the oxygen flow ratesproducing infinite selectivity and a roughness layer <10 nm thick)decreases in the order C₃H₇F>C₃H₆F₂>C₃H₅F₃. The data show thatincreasing F atoms on one terminal carbon significantly reduces theProcess Window. Only minor differences are observed in maximum SiN etchrate.

In FIG. 5, the ISR for C₃H₇F ranges from approximately 18.6 sccm O₂ to24 sccm O₂. In FIG. 6, the ISR for C₃H₆F₂ ranges from approximately 24sccm O₂ to 30 sccm O₂. In FIG. 7, the ISR for C₃H₅F₃ ranges fromapproximately 19.8 sccm O₂ to 22.8 sccm O₂. The calculated ISR valuesare presented in Table 1 in Example 8.

Example 3

Etch data using C₃H₆F₂ (1,1-difluoropropane), C₃H₆F₂(1,2-difluoropropane), C₃H₆F₂ (1,3-difluoropropane), and C₃H₆F₂(2,2-difluoropropane) were analyzed. All etch data was collected under200 W plasma RF power. The data compares isomers to demonstrate theimportance of F atom location in the etch gas molecule structure. Agraph of the SiN, SiO₂ and poly-Si etch rates versus O₂ flow (in sccm)for 1,1-difluoropropane (C₃H₆F₂) is shown in FIG. 6. A graph of the SiN,SiO₂ and poly-Si etch rates versus O₂ flow (in sccm) for1,2-difluoropropane (C₃H₆F₂) is shown in FIG. 8. In FIG. 8, the ISR for1,2-difluoropropane ranges from approximately 24 sccm O₂ to 29.4 sccmO₂. A graph of the SiN, SiO₂ and poly-Si etch rates versus O₂ flow (insccm) for 1,3-difluoropropane (C₃H₆F₂) is shown in FIG. 9. In FIG. 9,the ISR for 1,3-difluoropropane ranges from approximately 24 sccm O₂ to28.2 sccm O₂. A graph of the SiN, SiO₂ and poly-Si etch rates versus O₂flow (in sccm) for 2,2-difluoropropane (C₃H₆F₂) is shown in FIG. 10. InFIG. 10, the ISR for 2,2-difluoropropane ranges from approximately 15sccm O₂ to 18.6 sccm O₂. There is a noticeable progression ofdiminishing infinite selectivity ranges in the order1,1-difluoropropane>1,2-difluoropropane>1,3-difluoropropane, >2,2-difluoropropane.The calculated values are presented in Table 1 in Example 8. The datashows that C₃ molecules with more than one F atom yield larger infiniteselectivity ranges when the F atoms are on one terminal carbon.

Example 4

Etch data using C₂H₅F, 1-C₃H₇F (1-fluoropropane), and 1-C₄H₉F(1-fluorobutane) were analyzed. All etch data was collected under 200 Wplasma RF power. The data compares the effect of longer C_(x)H_(y)(where y=2x+1) branches on HFC molecules with a single F atom on aterminal carbon. A graph of the SiN, SiO₂ and poly-Si etch rates versusO₂ flow (in sccm) for fluoroethane (C₂H₅F) is shown in FIG. 3. A graphof the SiN, SiO₂ and poly-Si etch rates versus O₂ flow (in sccm) for1-fluoropropane (C₃H₇F) is shown in FIG. 5. A graph of the SiN, SiO₂ andpoly-Si etch rates versus O₂ flow (in sccm) for 1-fluorobutane (C₄H₉F)is shown in FIG. 23. In FIG. 23, the ISR for 1-fluorobutane ranges fromapproximately 31.8 sccm O₂ to 37.8 sccm O₂. There is a noticeableprogression of increasing infinite selectivity ranges in the orderfluoroethane<1-fluoropropane<1-fluorobutane. The calculated values arepresented in Table 1 in Example 8. The data shows that HFC moleculeswith one F atom on a terminal carbon have improved etch performance asthe C_(x)H_(y) (where y=2x+1) branch increases.

Example 5

A comparison of roughness on SiN layers etched with CH₃F and C₃H₇F wereanalyzed. Infinite SiN to poly-Si selectivity is obtained using flowrates of 50 sccm Ar, 30 sccm CH₃F, and 6 sccm O₂, with a 200 W plasma,and pressure of 5 Pa. After etching under these conditions, the SiNsurface has a thick roughness layer (˜82 nm), as shown in FIG. 11. Theroughness layer thicknesses for each etch condition in FIG. 2 is plottedin FIG. 12.

Infinite SiN to poly-Si selectivity is obtained using flow rates of 50sccm Ar, 20 sccm C₂H₃F₃, and 12.6 sccm O2, with a 200 W plasma, andpressure of 5 Pa. After etching under these conditions, the SiN surfacehas a thick roughness layer (˜82 nm), as shown in FIG. 13. The roughnesslayer thicknesses for each etch condition in FIG. 4 is plotted in FIG.14.

Infinite SiN to poly-Si selectivity is obtained using flow rates of 50sccm Ar, 20 sccm C₃H₇F, and 23.4 sccm O₂, with a 200 W plasma, andpressure of 5 Pa. After etching under these conditions, the SiN surfaceremains smooth, as shown in FIG. 15. Any surface roughness layer on theSiN following the C₃H₇F etch process is small enough that it is noteasily measured by the SEM cross section. The roughness layerthicknesses for each etch condition in FIG. 5 is plotted in FIG. 16.

The roughness layer on etched SiN is consistently lower in the infiniteselectivity range for C₃H₇F relative to CH₃F and 1,1,2-C₂H₃F₃. Surfaceroughness on etched SiN gate spacer layers is not desirable because theuneven removal of the SiN layer may result in more damage to theunderlying layer. The disclosed etch gases combined with the disclosedoxygen flow rates provide both infinite selectivity and a roughnesslayer <10 nm thick in the SAMCO10-NR reactive ion etcher, defined as theProcess Window in Table 1 in Example 8. One of ordinary skill in the artwill recognize that the oxygen flow rates producing both infiniteselectivity and a roughness layer <10 nm thick will vary depending onthe piece of equipment utilized.

Example 6

Fluoromethane, fluoroethane, 1,1,1-trifluoroethane, 1-fluoropropane,1,1-difluoropropane, and 2,2-difluoropropane were directly injected intoa quadrupole mass spectrometer (QMS) and data collected from 10-100 eV.The results are shown in FIGS. 17, 18, 19, 20, 21, and 22, respectively.Fragments from C₂H₅F, C₃H₇F, and C₃H₆F₂ (1,1-difluoropropane) havehigher H:F ratio than the fragments from CH₃F, 1,1,2-trifluoroethane,and 2,2-difluoropropane. The dominant fragment for C₃H₇F and C₃H₆F₂(1,1-difluoropropane) is C₂H₅. The fragmentation patterns for thesemolecules suggest that hydrofluorocarbons with hydrogen saturated carbonbranches create dense polymers on the substrate material which preventsetching or damage of the underlying substrate from the plasma species.

Example 7

Polymers were deposited by introduction into a RIE plasma reactionchamber at 10 sccm in the absence of other gases. The pressure in thechamber was set at 5 Pa. The plasma was set at 200 W. Deposition wasperformed on crystalline silicon substrates. Polymers were depositedfrom CH₃F at 23 nm/min. Polymers were deposited from C₂H₅F, C₂H₃F₃,C₃H₇F, and C₃H₆F₂ at 41 nm/min, 66 nm/min, 53 nm/min, and 45 nm/minrespectively.

Example 8

The following table (Table 1) summarizes the test results from theexamples above for multiple etch gases:

TABLE 1 1^(st) fragment 2^(nd) fragment Dep Rate Molecule¹ ISR³ PW⁴ at20 eV at 20 eV (nm/min)² CH₃F 1.8 0 CH₂F CH₃F 23 C₂H₅F 3.6 3.6 C₂H₄FCH₂F 41 C₂H₃F₃ 0.6 0 CHF₂ CH₂F 66 C₃H₇F 5.4 4.8 C₂H₅ C₂H₄ 53 1,1-C₃H₆F₂6 3.6 C₂H₅ C₃H₅F 45 1,2-C₃H₆F₂ 5.4 3.6 C₂H₄F C₂H₃F 55 1,3-C₃H₆F₂ 4.2 2.4C₂H₄F C₃H₄F 55 2,2-C₃H₆F₂ 3.6 1.2 C₂H₃F₂ C₂H₂F₂ 44 1,1,1-C₃H₅F₃ 3 1.2C₂H₅ C₃H4F₂ 49 C₄H₉F 6 6 C₃H₇ C₄H₈ 66Based on these results, the largest infinite selectivity ranges areobtained with high H:F ratio molecules. One of ordinary skill in the artwill recognize, and as is further evident from the figures, that theorder of fragment may differ at different energies. Among the high H:Fratio molecules, isomers which dissociate with a high density ofC_(x)H_(y) fragments have the largest infinite selectivity ranges. Thissuggests the preferred etch gas formula is C_(x)H_(y)F_(z) where z<3 andall F atoms are located on one terminal carbon. The polymer depositionrates do not correlate with the infinite selectivity ranges since C₂H₃F₃has the highest deposition rate, but the lowest infinite selectivityrange. The PW column shows that roughness significantly influences thedesired process range for lower H:F ratio molecules.¹C₂H₃F₃=1,1,2-trifluoroethane; C₃H₇F=1-fluoropropane;1,1-C₃H₆F₂=1,1-difluoropropane; 1,2-C₃H₆F₂=1,2-difluoropropane;1,3-C₃H₆F₂=1,3-difluoropropane; 2,2-C₃H₆F₂=2,2-difluoropropane;1,1,1-C₃H₅F₃=1,1,1-trifluoropropane; C₄H₉F=1-fluorobutane²10 sccmetching gas, 5 Pa and 200 W³ISR=Infinite selectivity range⁴PW=ProcessWindow=Range with infinite selectivity and less than 10 nm thickroughness layer

Example 9

FIG. 24 is an XPS graph providing the depth profile of a polymerdeposited layer on a poly-Si substrate. The polymer layer was depositedon poly-Si by operating a 1-fluoropropane plasma with the followingconditions: 20 sccm 1-fluoropropane, 22.2 sccm O₂, 50 sccm Ar, 200 W RFpower, 5 Pa process pressure, 2 minutes process duration. This set ofprocess conditions yields a SiN etch rate of approximately 56 nm/min anda SiN roughness layer of approximately 3 nm. Therefore, this processcondition lies within the PW for 1-fluoropropane.

The XPS depth profile was obtained by monatomic Ar sputtering and highresolution scanning of the Si, F, O, and C energy regions. The sputterdepth was calculated from a sputter rate of ˜8 nm/min. The C—Si data iscalculated from an auto-assigned peak at 283.02 eV. This energy isconsistent with C—Si relative to the adventitious C—C, C—H peak observedat 284.55 eV.

At the crossover point between C1s (C—C, C—H) and Si2p, the O1s is verylow (1.6%). The O1s quickly decreases to 0% within the next 3 nm. Theinitial analysis suggested that the poly-Si surface was free of carbideor oxide formation from the plasma process. Further advancement in ourXPS technique indicate that some carbide formation occurs as evidencedin FIG. 24.

Example 10

Etch data using CH₃F, C₂H₅F, and 1-F—C₃H₇ (1-fluoropropane) collectedunder 50 W plasma RF power were analyzed. The data compares etch ratesand infinite selectivity ranges for different gases operated with lowerRF power compared to the above examples. A graph of the SiN, SiO₂ andpoly-Si etch rates versus O₂ flow (in sccm) for fluoromethane (CH₃F) isshown in FIG. 25. In FIG. 25, the ISR for fluoromethane ranges fromapproximately 4.2 sccm O₂ to 5.4 sccm O₂. The roughness layerthicknesses for each etch condition in FIG. 25 is plotted in FIG. 28.The SiN roughness increases significantly in the ISR for CH₃F. As aresult, the process window (as defined above) for CH₃F etching at 50 Wis only 0.6. A graph of the SiN, SiO₂ and poly-Si etch rates versus O₂flow (in sccm) for fluoroethane (C₂H₅F) is shown in FIG. 26. In FIG. 26,the ISR for fluoroethane ranges from approximately 6.6 sccm O₂ to 7.8sccm O₂. The roughness layer thicknesses for each etch condition in FIG.26 is plotted in FIG. 29. The SiN roughness increases less abruptly inthe ISR for C₂H₅F compared to CH₃F. As a result, the process window forC₂H₅F etching at 50 W is 1.2. A graph of the SiN, SiO₂ and poly-Si etchrates versus O₂ flow (in sccm) for 1-fluoropropane (1-F—C₃H₇) is shownin FIG. 27. In FIG. 27, the ISR for fluoropropane ranges fromapproximately 16.8 sccm O₂ to 22.2 sccm O₂. The roughness layerthicknesses for each etch condition in FIG. 27 is plotted in FIG. 30.The SiN roughness does not increase in the ISR for 1-F—C₃H₇ until verylow O₂ flow rates. As a result, the process window for 1-F—C₃H₇ etchingat 50 W is a much wider 4.8 sccm O2. The SiN etch rates in FIGS. 25-27are significantly lower than the SiN etch rates in FIGS. 2, 3, and 5.The primary difference between the two sets of figures is the RF powerwhich is 200 W for FIGS. 2, 3, and 5, but only 50 W for FIGS. 24-26. Thedata show that infinite selectivity is attainable under low RF powerprocess conditions. Additionally, the SiN roughness negatively affectsthe CH₃F process window, while the C₂H₅F and 1-F—C₃H₇ process windowsare less affected by SiN roughness.

Example 11

Damage to the substrate as a result of SiN overetching can bedetrimental to device performance. It is therefore necessary to evaluateetch gases for their effects on substrate damage and consumption. FIG.31 shows the poly-Si consumption from an etch/ash cycle for a 200 W CH₃Fprocess. FIG. 32 shows the poly-Si consumption from an etch/ash cyclefor a 50 W CH₃F process. FIG. 33 shows the poly-Si consumption from anetch/ash cycle for a 50 W C₂H₅F process. FIG. 34 shows the poly-Siconsumption from an etch/ash cycle for a 50 W 1-F—C₃H₇ process. Blanketfilms of poly-Si with were processed with plasma conditions that yieldinfinite SiN/Poly-Si selectivity. The polymer films that coated thepoly-Si were then ashed in a 50 W, O₂ plasma. The difference between theinitial poly-Si film thickness and the final poly-Si film thickness wasplotted versus O₂ flow. The data points in the plot represent the fullrange of conditions where infinite selectivity is achieved for thespecific etch gas and process RF power. The poly-Si films processed with200 W, CH₃F plasmas show significant poly-Si loss (25-35 Å). The poly-Sifilms processed with 50 W, CH₃F plasmas show less poly-Si loss (21-27Å), but only for a very narrow range of O₂ flow. The poly-Si filmsprocessed with 50 W, C₂H₅F plasmas show even less poly-Si loss (17-21Å), and this low poly-Si loss is maintained for a wider range of O₂flow. The poly-Si films processed with a 50 W, 1-F—C₃H₇ plasma show theleast poly-Si loss (16-20 Å), and the widest range of O₂ flow with lowpoly-Si loss.

While embodiments of this invention have been shown and described,modifications thereof can be made by one skilled in the art withoutdeparting from the spirit or teaching of this invention. The embodimentsdescribed herein are exemplary only and not limiting. Many variationsand modifications of the composition and method are possible and withinthe scope of the invention. Accordingly the scope of protection is notlimited to the embodiments described herein, but is only limited by theclaims which follow, the scope of which shall include all equivalents ofthe subject matter of the claims.

What is claimed is:
 1. A silicon nitride plasma etching process foretching a SiN layer from a substrate, the method comprising: generatingon the SiN layer a SiN roughness layer of between approximately 0 nm andapproximately 10 nm thickness during the silicon nitride plasma etchingprocess by simultaneously introducing an oxidizer at a flow rate and anetch gas into a plasma reaction chamber containing the substrate, theetch gas having the formula C_(x)H_(y)F_(z), wherein x is 2-5, z is 1 or2, 2x+2=y+z, and a fluorine atom is located on a terminal carbon atom ofthe etch gas and the flow rate is selected to simultaneously yieldinfinite SiN to substrate selectivity and the SiN roughness layer ofbetween approximately 0 nm and approximately 10 nm thickness.
 2. Themethod of claim 1, further comprising determining the flow rate by (a)plotting a SiN etch rate for the etch gas on a Y axis versus oxidizerflow rate on a X axis; (b) plotting a substrate etch rate for the etchgas on a Y axis versus oxidizer flow rate on a X axis; and (c) selectingthe flow rate when the SiN etch rate is positive, the substrate etchrate is equal to or less than 0 nm/minute, and the SiN etch rate remainsapproximately the same with increasing oxidant flow rate.
 3. The methodof claim 1, further comprising determining the flow rate by optical orsurface topography analysis of a substrate having a partial SiN layerdeposited thereon to determine an oxidizer flow rate that (a) etches theSiN layer, (b) does not etch the substrate, and (c) does not generate onthe SiN layer a SiN roughness layer greater than 10 nm thick, whereinthe partial SiN layer does not cover the entire substrate.
 4. The methodof claim 1, further comprising ending the process by measuring a timewhen the substrate below the SiN layer begins to accumulate polymerdeposits.
 5. The method of claim 1, wherein the etch gas selectivelyreacts with SiN to form volatile by-products, further comprisingremoving volatile by-products from the plasma reaction chamber.
 6. Themethod of claim 5, further comprising ending the process by analyzingthe volatile by-products removed from the plasma reaction chamber todetermine when generation of volatile by-products ceases.
 7. The methodof claim 1, wherein the etch gas is selected from the group consistingof fluoroethane; 1-fluoropropane; 1,1-difluoropropane;1,2-difluoropropane, 1-fluorobutane, 1,1-difluorobutane,1,2-difluorobutane, 1-fluoropentane, 1,1-difluoropentane,1,2-difluoropentane, and combinations thereof.
 8. The method of claim 1,wherein z is
 1. 9. The method of claim 8, wherein the etch gas isfluoroethane.
 10. The method of claim 8, wherein the etch gas is1-fluoropropane.
 11. The method of claim 8, wherein the etch gas is1-fluorobutane.
 12. The method of claim 8, wherein the etch gas is1-fluoropentane.
 13. The method of claim 1, wherein z is
 2. 14. Themethod of claim 13, wherein x is
 3. 15. The method of claim 14, whereinthe etch gas is 1,1-difluoropropane.
 16. The method of claim 15, whereinthe etch gas is 1,2-difluoropropane.
 17. The method of claim 1, furthercomprising introducing an inert gas into the plasma reaction chamber,the inert gas being selected from the group consisting of He, Ar, Xe,Kr, and Ne.
 18. The method of claim 17, wherein the inert gas comprisesapproximately 25% by volume to approximately 95% by volume of a totalvolume of etch gas, oxidizer and inert gas introduced into the plasmareaction chamber.
 19. The method of claim 1, wherein the substrate isselected from the group consisting of silicon, polysilicon, siliconoxide, SiGe, Ge, GaAs, InGaAs, InP, InAs, or combinations thereof. 20.The method of claim 19, where in the substrate is polysilicon.
 21. Themethod of claim 1, wherein the oxidizer is selected from the groupconsisting of O₂, CO, CO₂, NO, N₂O, NO₂, SO₂, O₃, and combinationsthereof.
 22. The method of claim 21, wherein the oxidizer is O₂.